Adding a battery disconnect switch

[Kenneth Cole]

Hi Guys
Plan on adding a battery disconnect switch and wondering if it is necessary to use the size cable that is used on the batteries.
I my be wrong but it seems like overkill to me but I'm willing to learn.

Ken

 

[98G]

Absolutely.

Consider the amperage going through it as you start the truck...

 

[Milcommoguy]

Cable need to handle 200 amps with a peak and over 400 when hard starting / cranking (starter only add another 100 for glow plugs for intermittent requirement) (data from TM) It does seem a big over-kill and note it does go straight to the starter.

Other concerns, electric winch or not. Basic HMMWV electric loads are small (lights, heater, wipers, engine circuits) Radios light too ??

On the flip side charging system 60 - 100 - 200 amp alternator needs to be considered and if you need to push that kind of current back into the batteries (and if they could take it, something else is wrong) (slave starting a tank?) Distance and voltage drop need to be factored in too. (guessing it is a foot or two both leads to switch)

So for what ever the original design thoughts were I would stay with that size wire and an equally rated switch. (big bucks)

Here's a chart to make it more confusing. > https://www.wiringproducts.com/battery-cable

A little note that the shunt is a 1000 amp didy so they must of been thinking something big.

Arc welder on wheels, CAMO

 

[TOBASH]

There are several awesome posts regarding battery cut offs and placement of same posted within the last 2 months.

A little time and U will find 'em.

 

[lowell66dart]

I used a CAT disconnect switch that a member posted about here. Installed it in the front of the battery box panel in the passenger footwell. I had already removed all of the slave jumper stuff and one of those cables worked perfectly. I switched the negative side.

 

[dmetalmiki]

I use double throw Isolator switches. Like fitted to fire trucks etc.
All mine Isolate the POSITIVE, and the NEGATIVE terminals.
At least this fitting eliminates the discussion of which cable is better to disconnect!.
And ALL cabling is of the SAME size and capacity as the original truck battery /slave wires.

 

[JoeJrTheBarber]

When it comes to power cable on any vehicle bigger is always better...

 

[Mogman]

When installing a battery switch it does not matter if you switch the Pos or Neg, the battery is a series circuit with regards to the vehicle and ether will isolate the battery/batteries, both is just total overkill.
I prefer the neg because is something accidentally shorts one of the terminals to ground no big deal and I put switches on everything, saved my A$$ more than once.

 

[papakb]

Be careful if you have the 200 amp dual voltage alternator in your truck because a disconnect switch on the negative battery lead will still leave things grounded thru the mid battery connection. Remember that the TCM is powered thru that point and there is no shutoff for it. If your running the 60 amp system then you won't have any problems with a switch on the negative side. Mogman is right that you can disconnect either side (+ or _) or for that matter you can open the link between the batteries and do the same thing. I have a quick disconnect link in my 151 between the batteries that I remove when it's parked. It keeps the batteries from slowly discharging from stray leakage current and also acts as a security device.

 

[Retiredwarhorses]

When installing a battery switch it does not matter if you switch the Pos or Neg, the battery is a series circuit with regards to the vehicle and ether will isolate the battery/batteries, both is just total overkill.
I prefer the neg because is something accidentally shorts one of the terminals to ground no big deal and I put switches on everything, saved my A$$ more than once.

You should always use the ground to the rear battery for disconnect switch on a HMMWV, and military vehicles with 24v in general.

 

[Mogman]

You should always use the ground to the rear battery for disconnect switch on a HMMWV, and military vehicles with 24v in general.

That's easy enough to say but can you explain why? and why would a 24V military vehicle be any different that any other vehicle?

 

[frank8003]

https://cse.google.com/cse?cx=partn...steelsoldiers.com/search.php?searchid=6902070

 

[Mogman]

https://cse.google.com/cse?cx=partn...steelsoldiers.com/search.php?searchid=6902070

Have you tried that link? I do see some comments about disconnecting batteries but that was not the question
I'm not saying anyone is wrong, I just always like to challenge "common wisdom" The fact remains it is a series circuit, you could in fact put the switch between the batteries and get the same effect, in any case there is 24V difference in potential across the contacts when the switch is open so the switch requirements are the same no matter where you put the switch, as I said I prefer the neg side because it is safer in case something contacts the switch connections, but there are times when it makes more sense mechanically to install the switch on the positive side.

 

[papakb]

In a system with the 200A dual voltage alternator if you disconnect the ground leg (lead to the current shunt) the +24v side of the battery system will still be partially grounded thru the TCM so if you turned on anything that would normally be running on 24v it could still be seeing 12v. This voltage would be the high side battery grounding thru the TCM. This leg will not be turned off by a disconnect switch on the ground lead.

 

[Mogman]

Here is the danger of a ground side battery switch when using a 12V tap between the batteries, I have drawn these (rather crudely as I am not an artist) with current flow from positive to negative, electron flow is actually from neg to pos, but it is easier to understand looking at it as flowing from pos to neg.
for this scenario to cause a reverse polarity on the 12V electronics there must be a 24V load applied, in this example the headlights are turned on, but any load will cause this, ign. switch on, press the horn button etc.
As you can see when the battery switch is closed (normal operation) the current flow in the headlights and radio are from pos. to neg. and the whole world is happy. BUT if you look at the current flow with the battery switch open you can see the 12V equipment (radio in this example) has the current forced to flow backwards and that is a reverse polarity condition that can smoke that radio.

So a blanket assertion that a battery switch should ALWAYS be placed in the ground side of the batteries is false

 

[jnissen]

I disagree with your assertion the gear will see reverse polarity. It will still see the same 12V as earlier when the ground was hooked up. Just none of the 24VDC gear will see the proper voltage. The 12v gear will still be powered and not provide the desired cutoff.

I didn’t follow your drawing on current flow.


Sent from my iPhone using Tapatalk

 

[Retiredwarhorses]

Here is the danger of a ground side battery switch when using a 12V tap between the batteries, I have drawn these (rather crudely as I am not an artist) with current flow from positive to negative, electron flow is actually from neg to pos, but it is easier to understand looking at it as flowing from pos to neg.
for this scenario to cause a reverse polarity on the 12V electronics there must be a 24V load applied, in this example the headlights are turned on, but any load will cause this, ign. switch on, press the horn button etc.
As you can see when the battery switch is closed (normal operation) the current flow in the headlights and radio are from pos. to neg. and the whole world is happy. BUT if you look at the current flow with the battery switch open you can see the 12V equipment (radio in this example) has the current forced to flow backwards and that is a reverse polarity condition that can smoke that radio.

So a blanket assertion that a battery switch should ALWAYS be placed in the ground side of the batteries is false

you are free to believe whatever you want...

 

[Coug]

Anything properly designed to go into a 12/24v system on the 12V side of things should have some type of diode or other reverse polarity protection built in.

If you're adding 12V accessories to the system by just attaching them straight to the second battery, then yes, you could potentially see a -12V DC if there are other items turned on to create a loop from 12V to ground, then through a 24V circuit to the hot lead.

This is why you should always use a 24v to 12v converter to power 12V additions to your truck that weren't designed to be in a 12/24v system.

Now in reality, it's extremely unlikely that you'll have any issues, but in theory it is possible.

Edit: As they are set up from the factory, this is NOT an issue, everything is designed to have the ground lead be disconnected for anything to be done to the truck, it's ONLY when you start messing with the system and adding things NOT INTENDED for this type of system that you will possibly have any issues.

 

[Mogman]

In the 35 years I have been messing around with military vehicles I have seen this more than once and as a 40 year telecommunications technition doing component level repair I have the background in theory to back up my assertion.
Yes now that one can get a converter off of eBay for 20 bucks everyone should be using a converter but I assure you 10-15 years ago they did not exist in an affordable package and anyone wanting 12V tapped it off the batteries.
And there are folks out there right now doing it, they just might not admit to it.

I refurbish and and sell Motorola 2-way radios and just last year I replaced two Astro Spectra 110W mobiles before the dealer admitted they were in a S. TX law enforcement "command" vehicle where they were tapping off of the batteries for 12V and had a ground side battery switch, They had a generator to power the bulk of the communications gear, they just needed a mobile to use until they got to the "scene of the crime".

These radios have reverse polarity protection, that does not guarantee "protection"

ALSO I have read right here in SS.com where folks are planning to use a "tap" to keep memory live in 12V electronic devices

 

[Retiredwarhorses]

Anything properly designed to go into a 12/24v system on the 12V side of things should have some type of diode or other reverse polarity protection built in.

If you're adding 12V accessories to the system by just attaching them straight to the second battery, then yes, you could potentially see a -12V DC if there are other items turned on to create a loop from 12V to ground, then through a 24V circuit to the hot lead.

This is why you should always use a 24v to 12v converter to power 12V additions to your truck that weren't designed to be in a 12/24v system.

Now in reality, it's extremely unlikely that you'll have any issues, but in theory it is possible.

Edit: As they are set up from the factory, this is NOT an issue, everything is designed to have the ground lead be disconnected for anything to be done to the truck, it's ONLY when you start messing with the system and adding things NOT INTENDED for this type of system that you will possibly have any issues.

So AM General engineered the 12v side of the truck wrong....hmmm, I doubt it.